The present invention relates to a process for preparing 4-chlorobiphenyl by chlorination of biphenyl, isolation of the 4-chlorobiphenyl from the reaction mixture in high purity and 4-chlorobiphenyl having a low 3-chlorobiphenyl content.
4-Chlorobiphenyl is an important intermediate in the preparation of pharmaceuticals and crop protection products. It is therefore necessary that it can be obtained in a simple manner, inexpensively and in high purities.
Processes for preparing 4-chlorobiphenyl which require relatively complex precursors such as phenylboronic acid, 4-chloro-phenyl-diazonium salts, phenyl-magnesium bromide, 4-bromo-chlorobenzene, 4-iodo-chlorobenzene, phenyl triflate, 4-bromo-biphenyl and diphenyl-tetrazole compounds are known (cf Org. Chem., 62(10), 3405-6 (1997); Nippon Kagaku Kaishi, 1997 (2), 119-26; Tetrahedron, 52(21), 7201-20 (1996); Org. Chem. 57(1), 391-3 (1992) and J. Chem. Res., Synop. 1994 (6), 216-7). Even if the yields are sometimes above 90% and 4-chlorobiphenyl can be isolated readily in pure form and free of 3-chlorobiphenyl, the precursors can be prepared only by very complicated processes. For this reason, these processes for preparing 4-chlorobiphenyl are uneconomical and relatively unsuitable for use on an industrial scale.
Tetrahedron 52(26), 8863-6 (1996) describes the chlorination of biphenyl using 2 equivalents of tin tetrachloride and 1 equivalent of lead tetraacetate. The yield of 4-chlorobiphenyl is 70%. Nothing is said about the amount of 2- and 3-chlorobiphenyl formed and the degree to which they are separated off. The use of large amounts of lead tetraacetate and tin tetrachloride makes the process costly and uneconomical.
Angew. Chem., 103(12), 1687-9 (1991) describes the chlorination of biphenyl catalyzed by zeolites of the structure type LTL, with the potassium form (K-L) being mentioned as being particularly favorable. However, this publication is not directed at the preparation and isolation of 4-chlorobiphenyl, but rather the preparation and isolation of 4,4xe2x80x2-dichlorobiphenyl which can be crystallized from the product mixture.
It would be desirable to be able to prepare 4-chlorobiphenyl in a single step from biphenyl and chlorine. According to U.S. Pat. No. 1,890,427, this is possible by chlorination of biphenyl in chlorobenzene in the presence of metallic iron as catalyst. However, this gives a mixture comprising 4-chlorobiphenyl, 3-chlorobiphenyl, 2-chlorobiphenyl and unreacted biphenyl. The separation of this mixture is very complicated. For separating off one isomer, the reference describes crystallization from noneutectic mixtures above the freezing point of the eutectic. This means that separation of the entire mixture makes it necessary to carry out a complicated sequence of distillation and crystallization steps. Czech. Chem. Prum., 30(10), 529-32 (1980) confirms this. To achieve purities of greater than 99%, the latter reference describes the following steps to be carried out in succession for the separation of mixtures comprising 2-, 3- and 4-chlorobiphenyl: 1. distillation, 2. crystallization of the fractions from the distillation and 3. recrystallization of the fractions from the crystallization from ethanol.
The problems in isolating 4-chlorobiphenyl from reaction mixtures obtained in the chlorination of biphenyl explain why the complicated synthetic route indicated at the outset has been considered for the preparation of pure 4-chlorobiphenyl.
There is therefore still a need for a process which allows 4-chlorobiphenyl to be obtained in high purity in a simple manner from inexpensive starting compounds.
We have now found a process for preparing 4-chlorobiphenyl which is characterized in that biphenyl and chlorine are reacted in the presence of ring-chlorination catalysts and the reaction mixture obtained is subjected to fractional distillation.
Suitable ring-chlorination catalysts are the catalysts known for this purpose, e.g. the anhydrous chlorides of main groups 3, 4 and 5 and of transition groups 3 to 10 of the Periodic Table of the Elements, also the anhydrous chlorides of the rare earths. Preference is given to boron trichloride, aluminum trichloride and iron trichloride. Further suitable catalysts are heterogeneous aluminosilicates, e.g. sheet silicates such as montmorillonite and bentonite, heterogeneous aluminosilicates having amorphous, vitreous porous structures which can be prepared by coprecipitation or via cogels (see, for example, Ind. Eng., Chem. Res. 34, 421.33 (1995); J. F. Harrod and R. M. Laine, Applications of Organometallic Chemistry in the Preparation and Processing of Advanced Materials, Kluver Academic Publishers (1995), pp.27-46) and zeolites, e.g. those of the structure types xcexa9, X and L. Possible mobile ions in these zeolites are, for example, the elements of main groups 1, 2 and 3 and of transition group 3 of the Periodic Table of the Elements and the rare earths. Preference is given to H, Na, K, Rb, Cs, Ca, Mg, Sr, Ba, Sc, Y, La, Ce and Pr. Particularly preferred ring-chlorination catalysts are zeolites of the structure type L, very particularly preferably of the type K-L in which the mobile ions are from 80 to 100%, preferably from 90 to 100%, potassium ions.
The ring-chlorination catalyst can be used, for example, in amounts of from 0.2 to 20% by weight, preferably in amounts of from 2 to 12% by weight, based on biphenyl used.
The reaction according to the invention can be carried out at, for example, temperatures of from 0 to 120xc2x0 C., preferably temperatures of from 20 to 100xc2x0 C. The pressure during the reaction according to the invention is not critical and can be, for example, from 0.2 to 20 bar. It is preferably from 0.8 to 8 bar. Particular preference is given to carrying out the chlorination at atmospheric pressure.
The reaction according to the invention can be carried out batchwise, semibatchwise or continuously. Suitable batch reactors are, for example, stirred vessels, bubble columns and loop reactors. If the process is to be carried out continuously, this can be achieved by connecting a plurality of the abovementioned batch reactors in series. In principle, however, it is also possible to use residence tube reactors or reactors employing stationary catalyst beds.
The reaction according to the invention can be carried out in the presence or absence of a solvent and/or a cocatalyst. Suitable solvents are, for example, aprotic solvents which do not react with chlorine under the reaction conditions. Preference is given to chlorinated hydrocarbons, in particular methylene chloride. A suitable cocatalyst is, for example, chloroacetic acid. The chlorination according to the invention is generally carried out in such a way that from 30 to 90%, preferably from 10 to 70%, particularly preferably from 30 to 60%, of the biphenyl used are reacted in a single pass.
In the process of the invention, the reaction mixture present after the chlorination is separated into its components exclusively by means of a fractional distillation.
If a heterogeneous ring-chlorination catalyst, e.g. a zeolite, has been used, this is advantageously separated off mechanically, e.g. by filtration or centrifugation, prior to the distillation. If a soluble ring-chlorination catalyst such as iron trichloride has been used, this is advantageously deactivated and separated off, e.g. by addition of water and removal of the aqueous phase, prior to the distillation.
Before the distillation, a base can be added to the mixture to be distilled in order to destroy any residues having a catalytic activity and thus reliably to prevent transchlorinations during the distillation. Suitable bases for this purpose are, for example, sodium carbonate, calcium carbonate, sodium acetate and potassium hydrogencarbonate.
The columns for the fractionation of the reaction mixture from the chlorination, which may have been pretreated as described above, can have, for example, from 5 to 500, preferably from 10 to 100, particularly preferably from 20 to 50, theoretical plates. The manner in which this number of theoretical plates can be achieved by means of internals in the column is known to those skilled in the art. The columns are, for example, operated at a reflux ratio of from 1 to 30, preferably from 2 to 20.
The columns can be operated, for example, at temperatures at the bottom of from 100 to 300xc2x0 C., preferably from 150 to 250xc2x0 C., and at pressures beginning in the range from 0.5 to 3 bar and ending in the range from 3 to 300 mbar, preferably from 10 to 100 mbar.
The fractional distillation can be carried out batchwise, semibatchwise or continuously.
A continuous fractional distillation process can be carried out using, for example, three columns which are connected in series and are each equipped with stripping sections and enrichment sections. The first column gives unreacted biphenyl at the top, the second gives 2-chlorobiphenyl at the top and the third gives 4-chlorobiphenyl at the top.
A batchwise fractional distillation can be carried out using a column of the above-described type which is not equipped with a stripping section and an enrichment section.
If a solvent, e.g. methylene chloride, has been used, this is advantageously separated off before the actual fractional distillation, e.g. by distillation in a column installed upstream.
The chlorination according to the invention generally leads to reaction mixtures containing little 3-chlorobiphenyl. The ratio of 4-chlorobiphenyl to 3-chlorobiphenyl in the reaction mixture is, for example, above 100:1, frequently above 300:1 and particularly advantageously above 800:1. If it is, in an exceptional case, 100:1 or below, routine tests should be carried out to find a combination of reaction conditions, catalyst and possibly solvent in the case of which the desired low contents of 3-chlorobiphenyl can be obtained in the reaction mixture. The fractional distillation to be carried out according to the invention thus always makes it possible for the 3-chlorobiphenyl content of the 4-chlorobiphenyl isolated to be reduced appreciably in this way, even though the reduction may not be to below the detection limit.
Furthermore, the process of the invention makes it possible to obtain 4-chlorobiphenyl having a purity of 99.5% or more, frequently 99.95% or more, in a simple manner from simple starting materials and using simple auxiliaries. It is thus well suited for further processing to produce pharmaceuticals and crop protection products. The yields of 4-chlorobiphenyl in the process of the invention are above 90%.
The process of the invention has for the first time made available 4-chlorobiphenyl containing from 0.001 to 10‰, preferably from 0.01 to 1‰ and particularly preferably from 0.03 to 0.3‰, of 3-chlorobiphenyl. It is a typical feature of the process of the invention that it gives 4-chlorobiphenyl having these 3-chlorobiphenyl contents.
The present invention therefore also provides 4-chlorobiphenyl containing from 0.001 to 10‰, preferably from 0.01 to 1‰, particularly preferably from 0.03 to 0.3‰, of 3-chlorobiphenyl.